Experiments in this proposal seek to define how sigma3, one of the most abundant reovirus proteins in the virion and in the infected cell, interacts with viral protein mu1 to regulate viral entry and protein synthesis. In Aim I, molecular and biochemical approaches will be used to define the pH-sensitive and protease- dependent structural changes in capsid protein sigma3 that are required for reovirus particles to enter cells. Since little is known about how nonenveloped viruses breach cell membranes, these studies should elucidate fundamental cell entry mechanisms used by an important group of infectious agents. The results of these studies will contribute to an understanding of molecular mechanisms of viral persistence, since sigma3 mutations which influence virion disassembly are selected during the establishment of persistent reovirus infections. Experiments in Aim II address the fundamental question of how viral translation comes to predominate over cellular translation in reovirus- infected cells and how viral protein sigma3 affects this switch. Sigma3 belongs to a class of viral modulators which influence protein synthesis by binding dsRNA and preventing the activation of the dsRNA-dependent interferon-induced eIF2alpha kinase, PKR. However recent data indicate that sigma3 molecules from strains with distinct translational phenotypes are virtually identical in proposed dsRNA-binding sites. This suggests that in reovirus- infected cells, sigma3 activity is regulated. Several possible forms of regulation will be investigated. Biochemical and molecular approaches are proposed to test the hypothesize that sigma3 molecules differ in their affinity for mu1, and that mu1 influences the translational balance in infected cells by determining the level of sigma3 that is capable of binding dsRNA. Other molecular and genetic experiments explore an intriguing correlation between the ability of reovirus to shut off cellular translation and the cellular localization pattern of sigma3 molecules. These studies address the hypothesis that viral translation predominates in reovirus-infected cells because of a localized dsRNA-binding activity of sigma3. Results from studies in Aim II will contribute to an understanding of how viruses modulate the cellular translation machinery to favor their own replication.